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    SubjectRe: schedule() && prev/current (Was: [PATCH 3/3] Make get_current() __attribute__((const)))
    On 05/19, Peter Zijlstra wrote:
    > On Tue, 2010-05-18 at 23:22 +0200, Oleg Nesterov wrote:
    > > And, looking at this patch I think that schedule() can be simplified
    > > a little bit.
    > >
    > > "sched: Reassign prev and switch_count when reacquire_kernel_lock() fail"
    > > commit 6d558c3ac9b6508d26fd5cadccce51fc9d726b1c says:
    > >
    > > Assume A->B schedule is processing,
    > > ...
    > > Then on B's context,
    > > ...
    > > prev and switch_count are related to A
    > >
    > > How so? switch_count - yes, we should change it. But prev must be
    > > equal to B, and it must be equal to current. When we return from
    > > switch_to() registers/stack should be restored correctly, so we
    > > can do
    > What if schedule() got called at a different stack depth than we are
    > now?
    > I don't think we can assume anything about the stack context we just
    > switched to.

    Not sure I understand...

    OK. Firstly, we shouldn't worry about the freshly forked tasks, they
    never "return" from switch_to() but call ret_from_fork()->schedule_tail(),

    Now suppose that A calls schedule() and we switch to B. When switch_to()
    returns on B's context, this context (register/stack) matches the previous
    context which was used by B when it in turn called schedule(), correct?

    IOW. B calls schedule, prev == B. schedule() picks another task, prev
    is saved on B's stck after switch_to(). A calls schedule(), prev == A
    before context_switch(A, B), but after that switch_to() switches to
    B's stack and prev == B.


    I am looking into the git history now... and I guess I understand why
    reacquire_kernel_lock() uses current. Because schedule() did something

    prev = context_switch(prev, next); // prev == last


    reacquire_kernel_lock(current); // prev != current


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